The present invention relates to a process for producing printed circuits comprising a laminar support or board, on which is produced a plurality of electrically conductive tracks for connecting a plurality of electronic components. More particularly this invention relates to a novel technique for applying auxiliary conductive elements to the laminar support board, having the objective of increasing the conductivity of the tracks at specified points in the circuit, where the power components are located. The invention also relates to a printed circuit obtained by this novel process as well as to a loader of auxiliary conductive elements which is produced so as to be used with a process according to the invention.
The prior art teaches the producing electronic circuits with laminar supports or boards produced from xe2x80x9cvetronitexe2x80x9d or equivalent materials. The boards include, on one or both faces, one or more tracks of conductive material. Copper is a typical material for these tracks. These tracks are constructed by depositing a lamina of copper or equivalent material on the face of the laminar support and then carrying out a printing and etching procedure. Once the tracks have been constructed, components are added to the board. These electronic circuit components include both power components and logic components. Power components are generally characterised by their high power consumption and logic components are associated with low consumption. Both types of components are applied to the tracks produced on the board. The cross-section of the various conductive tracks is varied by the design architect in accordance with the amount of power consumed by the individual electronic components and the associated space available on the board. In this manner, the design can offer sufficiently limited resistance and avoid excessive heating. The cross-section can be increased by increasing the thickness of the copper lamina applied to the laminar support, or by increasing the width of the track. However, both of the dimensions of the transverse cross-section of a track cannot be increased at will for several reasons. As far as the thickness is concerned, since all the tracks are produced from the same conductive lamina, the latter must not be too thick so as not to increase the cost of the circuit beyond certain limits. Consequently it is important not to over-dimension the thickness of the tracks dedicated to the logic components in order to have sufficient conductivity in the tracks dedicated to the power components. Moreover, it is necessary to avoid the effects of the under-etching. This occurs where the conductive lamina from which the conductive tracks of the circuit are made is of great thickness. The width of the track is limited by the circuit layout and the spatial requirements for the board.
To obviate the design requirement limitations and drawbacks, the prior art also teaches the manually application of auxiliary conductive strips, consisting of copper slugs, which are soldered to the tracks. This is used for providing the power flow necessary for the high consumption of the power components connected to the board. The application of these auxiliary conductive strips currently requires an appreciable input of manual labour. The prior art teaches that each conductive strip is produced in a manner including shape, thickness, and other dimensions that are dedicated to the specific application. These conductive strips are soldered manually to the points at which an increase in the cross-section of the conductive track is required on account of the presence of power elements.
The objective of the present invention is the provision of a process for mounting auxiliary conductive elements on printed circuits, which is simpler, more economical and faster than the conventional systems.
Within the scope of this general objective, a particular objective of the present invention is the provision of a process which can be carried out using currently known machines and apparatus for mounting electronic components on printed boards.
A further objective of the present invention is the provision of a process which allows simple and economical production of printed circuits of different types without the necessity of dimensioning the auxiliary conductive elements in a specific and dedicated manner for each individual circuit.
A further objective of the present invention is the provision of a printed circuit with auxiliary conductive elements which is cheaper and simpler than conventional circuits.
These and further objectives and advantages, which will become clearly apparent to those skilled in the art from reading the following text, are achieved in substance by providing for the auxiliary conductive elements to be applied to the laminar support by means of an apparatus for applying SMD (Surface Mounted Device) technology components.
The invention is therefore based on the idea of manipulating the auxiliary conductive elements as if they were normal SMD technology electronic components. This allows mounting to be enormously simplified since the auxiliary conductive elements are also applied by the automatic machines in the same phase of application of the SMD components.
In practice, the process according to the invention can exhibit the following phases: producing at least one conductive track on the laminar support; applying a so-called xe2x80x9csolder-resistxe2x80x9d protective layer to the laminar support, leaving on the conductive track at least one zone substantially devoid of the said solder-resist layer; applying to the conductive track in the region of the said zone an adhesive for adhesively bonding the auxiliary conductive element; adhesively bonding the auxiliary conductive element and possible further SMD circuit components to the laminar support; and soldering the auxiliary conductive element and possible further SMD circuit components to the laminar support.
Electronic components can be mounted on the laminar support either on one face or on two faces, there normally being provision for the auxiliary conductive elements to be mounted on a single face together with the other SMD technology circuit components.
In a manner similar to what has been done hitherto in SMD technology, when components are mounted on both faces of the same laminar support, SMD technology circuit components, including the auxiliary conductive elements, will firstly be applied to one face by adhesive bonding. Once application has been done by adhesive, the drying of the adhesive will be carried out in an appropriate oven and there will be provision for the rotation of the support so as to allow mounting of the circuit components on the opposite face, by inserting the legs into the appropriate holes normally provided in the laminar support. Final soldering of the electronic components and of the auxiliary conductive elements, which completes the production cycle, will be carried out by a soldering procedure of known type, for example wave soldering with a tin or similar alloy.
The auxiliary conductive elements are preferably mutually identical so as to achieve the maximum standardisation of the process. However, the use also of different dimensions, or at least of a few different dimensional standards, on the same circuit, is not excluded. These may consist of metal pads of substantially rectangular outline of limited height, and may be produced from copper or some other highly conductive metal or metal alloy and may possibly be coated with a suitable metallic layer which facilitates the adhesive bonding and subsequent soldering. For example, the auxiliary conductive elements may have a forge tinning treatment or the like, with the objective also of preventing oxidation thereof during storage.
To allow automation of the procedure for mounting the auxiliary conductive elements, the invention advantageously provides for them to be loaded in strips of flexible material (for example plastic, cardboard or the like) furnished with a plurality of mutually identical recesses in which the auxiliary conductive elements are housed. These strips have the same structure as the normal loaders of SMD technology components used for mounting systems. It is thus possible to manipulate the auxiliary conductive elements as normal SMD technology components.
Further advantageous characteristics of the process according to the invention, of the printed circuit obtained therewith, and of the strips for loading the auxiliary conductive elements are indicated in the appended claims.